Enhanced melt-textured growth

ABSTRACT

A method for the enhanced melt-textured growth of superconducting crystals is disclosed for a sample having a first material capable of exhibiting superconducting properties. The sample is heated above a peritectic temperature of the first material, cooled below the peritectic temperature, and is subsequently subjected to a plurality of temperature spikes in which the sample is rapidly reheated above the peritectic temperature and recooled below the peritectic temperature to produce a superconducting crystalline structure substantially free of secondary nucleations. The resulting crystal is a superconducting crystalline structure comprising a plurality of bands formed in succession around a seed material. Each band has a non-uniform microstructure from an inner portion to an outer portion of the band.

TECHNICAL FIELD

The present invention relates to a method for enhancing themelt-textured growth of a superconducting crystalline structure and to anovel composition of matter for the resulting superconductingcrystalline structure.

BACKGROUND OF THE INVENTION

Melt-texturing is a process of controlling peritectic solidification toobtain a bulk, crystalline material with a high degree of latticeorientation. In the field of high-temperature superconductors,melt-texturing is used to eliminate high angle grain boundaries in thefinal crystalline structure. Grain boundaries substantially eliminatethe amount of current which can be carried without electrical resistancelosses. Superconductors have numerous uses including, but not limitedto, forming frictionless bearings or flywheels.

Conventional processes for melt-texturing superconductors begin with acomposition of matter termed a “123-phase,” a “123 YBCO” or a “Y-123”because the composition generally has the formula, YBa₂Cu₃O_(7-x).Superconductors based on the YBa₂Cu₃O_(7-x) system, where x #0.6, havebeen known since they were invented by IBM in 1986. These “hightemperature superconductors” are superconducting at temperatures wellabove absolute zero, for example, at 77K or higher. U.S. Pat. No.5,061,682, issued to Aksay et al., discloses a process for makingconductive and superconductive ceramics including Y₂BaCuO₅, YBa₂Cu₃O₇,and YBa₂Cu₄O₈, and is incorporated by reference herein.

Conventional melt-texturing processes generally heat the 123 YBCOmaterial above its peritectic temperature (approximately 1010° C. in airfor the 123 YBCO system) to decompose the 123 YBCO into its peritecticmixture which contains a second material termed a “211-phase” or“Y-211,” because the material generally has the formula Y₂BaCuO₅. TheY-123 also decomposes into a liquid rich in the Y (yttrium), Ba oxides,and Cu oxides. The mixture is subsequently supercooled slowly below theperitectic temperature of the 123 YBCO material. During this coolingperiod, the reverse reaction occurs wherein the Y-211 (Y₂BaCuO₅) reactswith the yttrium, the Ba oxides, and the Cu oxides in the liquid toreform 123 YBCO, which crystallizes at the supercooling temperature.Ideally, the 123 YBCO grains would nucleate uniformly such that theirorientations align perfectly. However, in the supercooling period, thereare often nucleations of grains with random orientations andspontaneous, secondary nucleations occur. These secondary nucleationsare referred to as parasitic grains because they consume materialavailable for the growth of desirable grains, create high angle grainboundaries upon their intersection with each other, and reduce thesuperconductive efficiency of the superconducting crystals.

Numerous processes are known and used to reduce these undesirable,secondary nucleations, however, such processes are incapable of haltingthese secondary nucleations once they begin to form. For example, U.S.Pat. No. 5,395,820, issued to Murakami et al., discloses a conventionalprocedure for a standard cooling cycle of 123-phase material. Theprocess is a variation of a conventional Melt-Powdering-Mett-Growthprocess. The process includes the steps of combining the 123-phase withBaCO₃ and CuO to prepare a mixed powder, heating the mixed powder,melting the mixed powder to form a molten material, rapidly solidifyingand cooling the molten material to form a solidified material,pulverizing the solidified material to form a fine powder, mixing 0.2 to2.0% by weight of a platinum powder such as PtBa₄Cu₂O_(y) with thepulverized fine powder, forming a body with the resultant mixture,heating the formed body to bring it to a partially-molten state, thenlastly, cooling the partially-molten formed body to finely disperse a211-phase and the platinum compound in a crystal of an oxidesuperconductor comprising a rare earth metal combined with Ba₂Cu₃O_(y),where y is a number sufficient to provide oxide superconductivity. Asuperconductor of 123-phase type is thus produced, and the 211-phase andplatinum compound are finely dispersed in a crystal of the 123-phase.Murakami, however, does not address any means of destroying secondarynucleations once they form. Furthermore, the process never increases thetemperature of the system above the peritectic temperature of the123-phase once crystallization of the 123-phase begins.

U.S. Pat. No. 6,046,139, issued to Blohowiak et al., also discloses amethod of making single 123 YBCO crystal superconductors. In the method,1-25 wt % of Y₂BaCuO₅ (211 YBCO material), 0.05-1.0 wt % Pt, and abalance of YBa₂Cu₃O_(7-x) (123 YBCO material) are combined. Pt isbelieved to limit the growth of the non-superconducting 211-phasecrystals. The resulting powder is pressed into a compact in a disk orother configuration. A seed crystal SmBa₂Cu₃O_(7-x), where x=1.6, is incontact with and is placed substantially parallel to the compact's topsurface. The compact is heated to a sintering temperature between 1010°C. and 1050° C. and held at that temperature for a time sufficient tofuse the seed crystal to the compact surface. The temperature is loweredat a rate of approximately 0.1-1.0° C. per hour. 123 YBCO crystal growthnucleates from the seed crystal as the materials cool. After nucleation,the compound is cooled at a rate of about 1-10° C. per hour to atemperature of approximately 950° C. 123 YBCO crystal growth radiatesfrom the nucleation site until the entire compact consists essentiallyof single crystal, single grain 123 YBCO body. The process, however,does not increase the temperature over the peritectic temperature oncecrystallization begins, and therefore, there is no means of destroyingsecondary nucleations once they form.

Other conventional melt-texturing processes attempt to limit thedetrimental secondary nucleations by slowly increasing the temperatureduring the growth phase. However, such processes are not very effectivein growing substantial superconducting crystalline structures becausethe temperature is never raised over the peritectic temperature afterthe initial heating of the material, and therefore, the processes areincapable of sufficiently destroying secondary nucleations once theyform. Moreover, this slow heating detrimentally slows growth of thecrystalline structure. For example, U.S. Pat. No. 6,171,390, issued toSatoh et al., discloses a method for preparing a large oxide crystallinematerial wherein a Y-123 oxide superconductive crystalline precursor isadded with seed crystals, supercooled below its peritectic temperature,and slowly heated while keeping the material in a supercooled conditionto promote crystal growth. While the method gradually increases thetemperature of the furnace during the growth stage to reduce secondarynucleations, this slow heating also reduces the crystal growth rate.Furthermore, the furnace temperature during the growth stage neverreaches the peritectic temperature of the system, therefore, the processis incapable of destroying secondary nucleations once they form.

Other such melt-textured growth processes have been disclosed but suchprocesses do not include raising the temperature above the peritectictemperature after the initial heating of the material during the growthphase of the superconducting crystal. “The Increase of the CriticalCurrent Density of YBa₂Cu₃O_(7-y) by a Modified Melt-Textured GrowthMethod” by Choi et al., Physica C 269, pp. 306-312 (1996) discloses aprocess for the melt-textured growth of superconducting crystals. Thearticle discloses a quick dip in temperature to reduce the number of 123nuclei after a mild increase in temperature. In the process, the 123material is sequentially: 1) heated to 1007° C. for 20 minutes; 2)raised above the peritectic temperature to 1050° C. for 30 minutes; 3)rapidly cooled over 5 minutes to a temperature below the peritectictemperature; 4) soaked at the cooled temperature for 2 to 8 minutes; 5)raised over 2 minutes to a higher temperature (which is still below theperitectic temperature); 6) soaked at that temperature for [t] hours; 7)cooled to 960° C. at a rate of 1 to 8° C./hour; and 8) cooled to roomtemperature at a rate of 50° C./hour. The process may reduce the numberof 123-phase nuclei during the initial slow-cooling process and alsoreduces the 211-phase particle size. However, the process does notdisclose raising the temperature above the peritectic temperature afterthe initial heating and during the growth stage to promote crystalgrowth.

Other melt-texturing processes utilize parameters other than temperatureto reduce growth of secondary nucleations. For example, U.S. Pat. No.5,856,277, issued to Chen et al., discloses a method of manufacturing atextured layer of a high temperature superconductor via decreasing thepartial pressure of O₂ in the atmosphere of the system to reduce theperitectic temperature of the system. The method includes providing anuntextured high temperature superconductor material having acharacteristic ambient pressure peritectic melting point, heating thesuperconductor to a temperature below the peritectic temperature,establishing a reduced O₂ atmospheric pressure causing a reduction ofthe peritectic melting point which causes melting from an exposedsurface of the superconductor, and subsequent raising of the partialpressure of the O₂ in the atmosphere (while raising temperature) tocause solidification of the molten superconductor in a textured surfacelayer. However, once the O₂ partial pressure is increased to beginsolidification of the superconductor, the process does not utilize avariation in oxygen concentration to selectively destroy detrimental,secondary nucleations that have formed in the process.

Additionally, the above mentioned processes do not disclose a means ofgrowing numerous crystals in a single run. Typically, the aboveprocesses require expensive specialized ovens in which only one areawithin the oven is optimized for rapid 123-phase crystal growth withminimal secondary nucleations. Furthermore, the above-mentionedprocesses do not give rise to a resulting 123-phase crystallinestructure having differing superconductivity properties within resultinggrowth bands of the crystalline structure.

Therefore, it would be desirable to provide a method for enhancing themelt-textured growth of a superconducting crystalline structure toproduce a superconducting crystalline structure substantially free ofsecondary nucleations and having superior characteristics.

SUMMARY

In view of the deficiencies described above, it is an object of thepresent invention to provide a method for enhancing the melt-texturedgrowth of a superconducting crystalline structure to produce asuperconducting crystalline structure substantially free of secondarynucleations and having superior characteristics.

The present invention discloses a method for enhancing melt-texturedgrowth of a superconducting crystalline structure wherein a samplecomprising a first material capable of exhibiting superconductivityproperties is subjected to a plurality of temperature spikes above andbelow the peritectic temperature of the first material such thatdetrimental secondary nucleations are destroyed and a superconductingcrystalline structure is formed.

There is provided a sample with a first material which is capable ofexhibiting superconducting properties when subjected to melt-texturedgrowth. Preferably, the first material is a metal oxide, such as a rareearth metal oxide having the formula REBa₂Cu₃O_(7-x) (RE-123), whereinRE is at least one rare earth metal selected from the group consistingof Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb, and wherein x is anumber sufficient to provide oxide superconductivity. Preferably, thesample is compressed into a pellet. A seed material is preferably placedon a top portion of the pellet to form a center from which the firstmaterial crystals grow. The seed material is also preferably a rareearth metal oxide material of the formula, REBa₂Cu₃O_(7-x), but the REis selected such that the seed material has a higher peritectictemperature, referred to herein as a “seed peritectic temperature,” thanthe peritectic temperature of the first material because it is desirablethat the seed material not melt during the subsequent heating of thesample.

The sample is heated to a temperature of at least 10° C. above theperitectic temperature of the first material, and preferablyapproximately 20° C. above the peritectic temperature, which brings thefirst material to a semi-molten state. Where the first material is aRE-123 material (REBa₂Cu₃O_(7-x)), virtually all the RE-123 material ismelted into its peritectic mixture which comprises a solid called“RE-211” having the formula RE₂BaCuO₅, wherein RE is at least one rareearth metal selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho,Er, Nd, La, Pr, and Yb. The RE-123 material also melts into a liquidrich in the rare earth metal, a Ba oxide, and a Cu oxide. Subsequent tothe initial heating over the peritectic temperature, the pellet iscooled such that the RE-211 and liquid react to reform RE-123 whichbegins to crystallize and propagate from the seed material. The repeatednucleation of RE-123 grains forms the superconducting crystallinestructure. The pellet is preferably cooled to a temperature between 10°C. to 30° C. below the peritectic temperature of the first material, andpreferably 20° C. below the peritectic temperature, for RE-123formation.

The pellet is subsequently subjected to at least one temperature spikeabove and below the peritectic temperature of the first material. Thepellet is first heated above the peritectic temperature to destroyRE-123 crystals which are misaligned and have a high angle grainboundary. These misaligned RE-123 crystals are remelted into solidRE-211 plus the liquid when the temperature is raised above theperitectic temperature. This raised temperature thus increases theconcentration of RE-211 into solution and causes more of the rare earthmetal (RE) to be deposited out as RE-123 when the pellet is recooled.The liquid phase also increases in Ba and Cu oxides, which increases thesolubility of the rare earth metal in the liquid. Sufficient RE-123remains after the initial temperature increase with which to resumecrystal growth thereon because the temperature increase is for arelatively short duration. Once the sample is recooled, RE-211 againreacts with the liquid to continue formation of the superconductingcrystalline structure. By increasing the number of temperature spikesand total furnace growth time, larger single crystals or multi-seededbulk shapes can be formed. The sample must finally be oxygenated orannealed in order for the resulting crystalline structure to be acquiresuperconducting characteristics. One preferred manner in which tooxygenate the sample is to cool the sample to a temperature between 400°C. to 500° C. and place the sample in an oxygen rich environment. It isunderstood that “having a first material exhibiting superconductingproperties when subjected to melt-textured growth,” as used herein,incorporates the concept that the material may need to be oxygenatedbefore it acquires superconductive properties.

The resulting crystalline structure comprises a plurality of bandsformed in succession and propagating from a seed material wherein eachband has a non-uniform microstructure from an inner portion of the bandto an outer portion of the band. Each of the bands are defined as thearea between successively nested boundaries. Specifically, each of thebands comprises a superconducting material occupying a space betweensuccessively nested boundaries which are configured as hollowrectangular polyhedrons. The hollow rectangular polyhedron is a shell ofa polyhedron in which all opposing sides are parallel and itscross-section is rectangular. Each of the successively nested boundariesat least partially envelop the previous boundary. For example, when aseed material is placed on top of the sample, the successively nestedboundaries will envelop the previous boundary on five sides. When a seedmaterial is located in the center of the sample, the successively nestedboundaries will envelop the previous boundaries on all sides to theextent the boundaries are contained within the pellet.

The resulting crystalline structure preferably contains a first metaloxide material such as RE-123, and a second metal oxide material such asRE-211. Within each band, there is a higher concentration of RE-123 onan inner portion of each band than on an outer portion of the band.There is also a higher concentration of RE-211 on the outer portion ofeach band than on the inner portion of each band because the RE-123formation quickens once the step of recooling the sample during atemperature spike begins. From band to band, the concentration of RE-211present at the inner portion of each succeeding band is greater than theconcentration of RE-211 present at the inner portion of the immediatelypreceding band. Also, the concentration of RE-211 present at the outerportion of each succeeding band is greater than the concentration ofRE-211 present at the outer portion of the immediately preceding band.The precise amount of RE-211 in the final crystalline structure, and thesize and width of each band, can be controlled by varying the durationand magnitude of the recooling step within the temperature spike.

Other embodiments of the invention are disclosed which reduce the numberof detrimental secondary nucleations. In one embodiment, a furnace inwhich the ambient environment can be controlled reduces the peritectictemperature of the sample by reducing the oxygen content in the ambientenvironment. The oxygen content is first reduced in the atmosphere todecompose the RE-123 from the outside of the pellet much like a spike intemperature. Oxygen is then reintroduced into the ambient environment tobring the sample back into an undercooled status where RE-123 crystalsform. The sample is then subjected to a series of oxygen content spikeswhich are similar to the temperature spikes of the previous embodimentsexcept a change in oxygen content is substituted for a change intemperature. The oxygen content is alternately decreased and increasedto destroy secondary nucleations and promote growth of a superconductingcrystalline structure.

Other features and advantages of the invention will be apparent from thefollowing detailed description taken in conjunction with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the temperature sequence of apreferred embodiment of the present invention.

FIG. 2 is a graphical representation of the temperature sequence ofanother preferred embodiment of the present invention.

FIG. 3 is a graphical representation of the temperature sequence of aprior art melt-textured growth method.

FIG. 4A is a top view of a sample showing a seed before crystallinegrowth in a preferred embodiment of the present invention.

FIG. 4B is a top view of a sample showing crystalline growth after onetemperature spike in a preferred embodiment of the present invention.

FIG. 4C is a top view of a sample showing crystalline growth after twotemperature spikes in a preferred embodiment of the present invention.

FIG. 4D is a top view of a sample showing crystalline growth after threetemperature spikes in a preferred embodiment of the present invention.

DETAILED DESCRIPTION

While this invention is susceptible of embodiments in many differentforms, there is shown in the drawings and will herein be described indetail preferred embodiments of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspect of the invention to the embodiments illustrated.

The present invention is a method for enhancing the melt-textured growthof a superconducting crystalline structure wherein a sample comprising afirst material capable of exhibiting superconducting qualities issubjected to a plurality of temperature spikes above and below theperitectic temperature of the first material such that detrimentalsecondary nucleations are destroyed and a superconducting crystallinestructure is formed.

There is provided a sample with a first material which is capable ofexhibiting superconducting properties when subjected to melt-texturedgrowth. Preferably, the first material is a metal oxide, such as a rareearth metal oxide having the formula REBa₂Cu₃O_(7-x) (RE-123), whereinRE is at least one rare earth metal also selected from the groupconsisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb, wherein xis a number sufficient to provide oxide superconductivity, for example,0.6 or less, or any other suitable variable. In one preferredembodiment, the rare earth metal is Y. It is understood that thechemical formulation of the RE-123 is presently not necessarily preciseand that the subscripts given represent at least approximate ratios ofthe respective elements. Preferably, the sample is compressed into apellet via a compressor or any other suitable means. The first materialis preferably pressed into a single die which is hexagonal incross-sectional shape, but may be of any other suitable shape. In onepreferred embodiment, the single die is 2″ in diameter measured across aface of the die.

A seed material is preferably placed on a top portion of the pellet toform a center from which the first material crystals will propagate.However, the seed can optionally be positioned anywhere within thepellet, such as near its center. The seed material is preferably a rareearth metal oxide material having a chemical formula REBa₂Cu₃O_(7-x),wherein RE is at least one rare earth metal also selected from the groupconsisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb, and whereinx is a number sufficient to provide oxide superconductivity. The RE(rare earth metal) of the seed material is preferably selected such thatthe seed material has a higher seed peritectic temperature than theperitectic temperature of the first material because it is desirablethat the seed material not melt during the subsequent heating of thesample. In one preferred embodiment, the rare earth metal of the seedmaterial is Sm. It is understood that while the seed material may be adifferent material than the first material, the seed material optionallycomprises the first RE-123 grains which form during the cooling steps ofthe method from which growth of the crystalline structure propagates.

The sample is heated to at least 10° C. above the temperature above aperitectic temperature of the first material, and preferablyapproximately 20° C. above the peritectic temperature, which brings thefirst material to a semi-molten state. Where the first material is aRE-123 material (REBa₂Cu₃O_(7-x)), virtually all the RE-123 material ismelted into its peritectic mixture which comprises a solid called“RE-211” having the formula RE₂BaCuO₅, wherein RE is at least one rareearth metal also selected from the group consisting of Y, Sm, Eu, Gd,Dy, Ho, Er, Nd, La, Pr, and Yb. The RE-123 material also melts into aliquid rich in the rare earth metal, a Ba oxide, and a Cu oxide.Subsequent to the initial heating over the peritectic temperature, thepellet is cooled such that the RE-211 and liquid react to reform RE-123,which begins to crystallize and propagate from the seed material. Therepeated nucleation of RE-123 grains forms the superconductingcrystalline structure. Preferably, the pellet is cooled to a temperature10 to 20° C. below the peritectic temperature for RE-123 formation.

The pellet is subsequently subjected to at least one temperature spikeabove and below the peritectic temperature of the first material. Thepellet is first heated above the peritectic temperature to destroyRE-123 crystals which are misaligned and have high angle grainboundaries. These misaligned RE-123 crystals are remelted into itsperitectic mixture comprising solid RE-211 and liquid when thetemperature is raised above the peritectic temperature of the firstmaterial. The raised temperature increases the concentration of RE-211into solution and causes more of the rare earth metal (RE) to bedeposited out as RE-123 when the pellet is recooled. The liquid phasealso increases in Ba and Cu oxides, which increases the solubility ofthe rare earth metal in the liquid. Sufficient R-E-123 remains after theinitial temperature increase with which to resume crystal growth thereonbecause the temperature increase is for a relatively short duration.Once the sample is recooled, RE-211 again reacts with the liquid tocontinue formation of the superconducting crystalline structure. Byincreasing the number of temperature spikes and total furnace growthtime, larger single crystals or multi-seeded bulk shapes can be formed.

If there were no secondary nucleations in the resulting crystallinestructure, only uniform crystals of RE-123 would form on the seedmaterial. Some of the RE-123 would trap smaller RE-211 grains,especially away from the seed material. However, secondary nucleationswith high angle grain boundaries often form away from the seed materialwhich lower the superconductivity of the resulting crystallinestructure. The subsequent steps of the present invention halt the growthof the secondary nucleations and allow RE-123 to grow through theremnants of these secondary nucleations, by subjecting the pellet to atleast one temperature spike.

In the temperature spike sequence of the method, the pellet is firstheated above the peritectic temperature to destroy the RE-123 crystalswhich are misaligned and have a high angle grain boundary. Specifically,the increase in temperature remelts the RE-123 secondary nucleationsback into its peritectic mixture and the concentration of RE-211 insolution is increased such that more of the rare earth metal (RE) can bedeposited out as RE-123. The liquid phase also increases inconcentration of Ba oxides and Cu oxides, which increase the solubilityof the rare earth metal in the liquid. In a preferred embodiment, thetemperature is increased to a reheating temperature of 10° C. over theperitectic temperature of the first material at a rate of 1° C./min.During the reheating phase of the temperature spike, much of the RE-123material that crystallized in the initial cooling phase remelts suchthat crystallization easily resumes once the temperature is decreasedagain. Enough RE-123 remains after the initial temperature increase withwhich to resume crystal growth thereon because the temperature increaseis for a very short duration.

The pellet is lastly recooled to a recooling temperature which ispreferably between 10° C. and 35° C. below the peritectic temperature ofthe RE-123 material wherein crystallization of the RE-123 materialresumes. Preferably, the pellet is recooled at an initial rate of 0.5°C./min. As grains of the RE-123 material begin to crystallize, a portionof the available RE-211 is not reacted and a number of such RE-211grains are trapped between RE-123 grains as the RE-123 crystallizes.These trapped RE-211 particles are non-conducting and thus createpinning centers with the formed superconducting RE-123 crystallinestructure such that a magnetic field can rotate around the outerperimeter of the resulting crystal. Since the amount of liquid decreaseswith each subsequent temperature spike, the amount of RE-211 increases,and thus the rate of RE-123 crystallization further decreases during thecooling period of each subsequent spike. However, this decrease inliquid and decrease in RE-123 formation with each subsequent temperaturespike can be countered if desired by extending the cooling periodduration and temperature of the cooling period. Therefore, when using aplurality of temperature spikes, the cooling portion of each temperaturespike can reach a successively cooler temperature to promote RE-123crystal growth. In a preferred embodiment of the invention, eight suchtemperature spikes are used culminating in a temperature spike whichcools to a recooling temperature 35° C. below the peritectic temperatureof the first material. FIG. 1 is a graphical representation of thetemperature sequence of a prior art melt-textured growth method. FIG. 2and FIG. 3 are each a graphical representation of a temperature sequenceof a preferred embodiment of the present invention.

By increasing the number of temperature spikes and total furnace growthtime, larger single crystals or multi-seeded bulk shapes can be formed.After being subjected to at least one temperature spike, the sample isoxygenated or annealed in order for the resulting crystalline structureto acquire superconducting characteristics. One preferred manner inwhich to oxygenate the sample is to cool the sample to a temperaturebetween 400° C. to 500° C. and place the sample in an oxygen richenvironment. It is understood that “having a first material exhibitingsuperconducting properties when subjected to melt-textured growth,” asused herein, incorporates the concept that the material may need to beoxygenated or annealed before it acquires superconductive properties.Likewise, it is understood that the terms “superconductive crystallinestructure,” as used herein, refer to the structure of the materialproduced via the disclosed process, both before and after an oxygenationhas occurred, regardless of whether the structure has acquiredsuperconducting characteristics at that stage.

Typically, the resulting crystalline structure measures between 2-4inches in diameter and between 0.5-2 inches in thickness and the methodrequires approximately 4-8 days to complete. The resultingsuperconducting crystalline structure comprises a plurality of bandsformed in succession and propagating from a seed material, wherein eachband has a non-uniform microstructure from an inner portion of the bandto an outer portion of the band. Each of the bands are defined as thearea between successively nested boundaries. Specifically, each of thebands comprises a superconducting material occupying a space betweensuccessively nested boundaries being configured as hollow rectangularpolyhedrons. The hollow rectangular polyhedron is a shell of apolyhedron in which all opposing sides are parallel and itscross-section is rectangular. Each of the successively nested boundariesat least partially envelop the previous boundary. For example, when aseed material is placed on top of the sample, the successively nestedboundaries will envelop the previous boundary on five sides. When a seedmaterial is located in the center of the sample, the successively nestedboundaries may envelop the previous boundaries on all sides to theextent the boundaries are contained within the pellet.

Preferably, the superconducting crystalline structure within each bandcomprises a first metal oxide material such as REBa₂Cu₃O_(7-x) (RE-123),wherein RE is at least one rare earth metal selected from the groupconsisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb and x is anumber capable of providing oxide superconductivity. The seed materialis also preferably a material of the same formula, REBa₂Cu₃O_(7-x),wherein RE is at least one rare earth metal selected from the groupconsisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb and x is anumber capable of providing oxide superconductivity. However, the rareearth metal of the seed material is selected to provide the seedmaterial with a higher seed peritectic temperature than the peritectictemperature of the first material. It is understood that while the seedmaterial may be a different material than the first material, the seedmaterial may also comprise the first grains of the first material whichform during the growth of the crystalline structure. The superconductingcrystalline structure further comprises particles of a second metaloxide material having a formula of RE₂BaCuO₅ (RE-211), wherein RE is atleast one rare earth metal selected from the group consisting of Y, Sm,Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb.

When the RE-123 material is heated above its peritectic temperature inthe formation phase, the RE-123 material decomposes into its peritecticmixture comprising a solid RE-211 material and a liquid containing therare earth element, and Cu and Ba oxides. When the temperature is thencooled below the peritectic temperature, the RE-211 and liquid react toreform RE-123 and RE-123 crystallizes. Therefore, during eachtemperature spike, when the temperature is raised above the peritectictemperature, RE-123 will decompose into RE-211 and when the temperatureis dropped below the peritectic temperature, RE-211 will aid inreforming RE-123. The width of composition of the bands can becontrolled via varying the recooling temperature and the reheatingtemperature of the temperature spikes.

For example, when the pellet is first cooled, the crystallization ofRE-123 will be very quick and the first band will quickly form becausethere is a multitude of reactable RE-211 and liquid. With eachsubsequent temperature spike, the amount of liquid is decreased and thusthe rate of RE-123 crystallization decreases during the recooling periodof each subsequent spike. Therefore, the bands of the crystallinestructure will decrease in width in at least one dimension as each ofthe bands form from the seed material if the recooling temperature iskept constant after each spike.

Furthermore, within each band, there is a higher concentration of RE-123on an inner portion of each band than on an outer portion of the band.There is also a higher concentration of RE-211 on the outer portion ofeach band than on the inner portion of each band because the RE-123formation is initially quick once the recooling begins. Additionally, asthe RE-123 crystals form, RE-211 is pushed in front of RE-123 growth.From band to band, the concentration of RE-211 present at the innerportion of each succeeding band is greater than the concentration ofRE-211 present at the inner portion of the immediately preceding band.Also, the concentration of RE-211 present at the outer portion of eachsucceeding band is greater than the concentration of RE-211 present atthe outer portion of the immediately preceding band. The resultingcrystalline structure further comprises impurities, also termed“porosities,” which are concentrated in various areas and which arenon-uniform from the inner portion to the outer portion of each band.

A superconducting element formed in this fashion is capable of multiplepossible applications, including a switch having numerous distinctoutputs from activation of various bands in response to varying currentlevels in a control conductor.

The decrease in liquid, rare earth metal concentration, and in RE-123formation with each subsequent temperature spike can be countered byextending the cooling period duration and temperature of the coolingperiod. Therefore, bands of substantially the same size can be formed byappropriately dropping the recooling temperature with each subsequentspike and also by extending the cooling period. If the cooling periodsare not significantly extended nor the cooling temperature dropped, thebands will decrease in width in at least one dimension in a directionmoving away from the seed material. If the cooling periods are adjusted,the bands may be made substantially equivalent in width in at least onedimension if desired.

Referring to FIG. 4A, the seed material 10 is placed in the middle of acompacted powder pellet 20. Referring to FIG. 4B, the figure representsthe beginning of the seeded domain growth. A secondary nucleation 30 isrepresented in the lower portion of the pellet 20. The secondarynucleation 30 would be seen as a more reflective spot in an otherwisedull black pellet 20. FIG. 4C illustrates a pellet 20 that has beensubjected to a temperature spike, however an additional distinct band 40has been added. FIG. 4D illustrates a pellet 20 that has been subjectedto an additional temperature spike. Due to the temperature spikes, thesecondary nucleation 30 is stopped and growth from the seed 10 growsthrough the remnants of these parasitic nucleations 30. Each successiveband 40 is more reflective than the previous band 40 in a direction awayfrom the seed material 10.

Other embodiments of the invention are disclosed which reduce the numberof detrimental secondary nucleations. In one embodiment, a furnace inwhich the ambient environment can be controlled reduces the peritectictemperature of the sample by reducing the oxygen content in the ambientenvironment. The oxygen content is first reduced in the atmosphere todecompose the RE-123 from the outside of the pellet much like a spike intemperature. Oxygen is then reintroduced into the ambient environment tobring the sample back into an undercooled status where RE-123 crystalsform. The sample is then subjected to a series of oxygen content spikeswhich are similar to the temperature spikes of the previous embodimentsexcept a change in oxygen content is substituted for a change intemperature. The oxygen content is alternately decreased and increasedto destroy secondary nucleations and promote growth of a superconductingcrystalline structure. Alternatively, the peritectic temperature couldbe reduced by reducing the atmospheric pressure and the peritectictemperature could be increased by increasing the atmospheric pressurewithin the ambient environment.

In another embodiment of the invention, the pellet does not contain aseed material having a different chemical formula from the firstmaterial. In this embodiment, the pellet is subjected to the same seriesof steps of the method to form the superconducting crystallinestructure. The initial RE-123 crystals which form provide a seed fromwhich other RE-123 crystals propagate. Since the present inventionallows for the selective destruction of secondary nucleations, themethod is also conducive to non-seeded texturing methods. For example,when the pellet is ready to be subjected to at least one temperaturespike, the pellet could be visually inspected to determine the optimumrecooling temperature, reheating temperature, rate of recooling, andrate of reheating by monitoring band growth.

In yet another embodiment of the present invention, the temperature ofthe system may be increased locally via a laser or microwave or by anyother suitable means. Rather than heating the entire pellet to limit afew secondary nucleations, heat is directed to only specific locations.For example, one could focus a laser across the surface of a pelletavoiding any desirable growth while heating other areas and leaving theminhospitable for competing growth.

In yet another embodiment of the present invention, a modifier may beadded to the pellet as a dopant/modifier. The modifier is generallyplatinum, cerium, or silver, but other elements could be added includinggold, ruthenium or palladium. RE-123 accepts many elements as dopantssuch as calcium, lithium, strontium, uranium, zinc, and zirconium. Themodifiers enhance the melting of the RE-123 material and thus thedestruction of secondary nucleations and also may act as pinning centersor diminish any damaging effects of the temperature spikes. The dopantsenhance the pinning of superconducting currents and improve propertiesof the resulting superconducting crystalline structure.

EXAMPLE 1

Thirty grams of commercially available powder of nominal compositionY_(1.254)Ba_(2.127)Cu_(3.127)O_(7.635-x)+0.25 wt % Pt was pressed at5,000 lbs. in a 1.15″ cylindrical steel die. After forming, the powderwas further compacted in a Cold Isostatic Press (CIP) to a pressure of25 kpsi. The pellet was then sintered at 900° C. for 8 hours. A pelletdensity of >6.0 g/cc is optimum.

Following the sintering of the pellet, a seed material having theformula SmBa₂Cu₃O_(7-x) (“Sm-123”) was placed on top of the dense pelletand placed in a furnace for heat treatment/growing. The growth furnacewas heated to between 1025-1045° C. for 75 minutes to bring the pelletto a semi-molten state wherein substantially all the RE-123 was meltedwithout melting the Sm-123 seed material. The furnace was then cooledbelow the peritectic temperature of RE-123 (approximately 1010° C.) tosubject the pellet to a supercooled condition and to promote orientedgrowth starting at the seed. After 1-12 hours of growing at temperaturesbelow the peritectic temperature the pellet was subjected to eighttemperature spikes wherein the furnace setpoint was increased to 1020°C. and quickly cooled to approximately 20° C. below the peritectictemperature to 990° C. Optimum conditions resulted in 1-3 mm of newgrowth from each undercooling before a spike in temperature wasadvisable to eliminate secondary nucleations.

EXAMPLE 2

120 grams of powder of nominal compositionY_(1.254)Ba_(2.127)Cu_(3.127)O_(7.635-x)+0.25 wt % Pt was pressed at2,000 lbs in a hexagonal steel die. The die size resulted in a powdercompact with about 1.78″ of material between parallel faces 1″ thick.After forming, the powder compact was further compacted in a CIP to apressure of 25 kpsi. In this embodiment, the compact was not sinteredprior to growth heat treatment.

A Sm-123 seed crystal was placed on top of the pellet and placed in abox-furnace for heat treatment/growing. The furnace was heated to 1030°C. at a rate 3° C./min and held for 95 minutes followed by 1.5° C./mincooling to 1006.2° C. The material was then slowly cooled until growthcould be easily observed around 995° C.

A series of temperature spikes were programmed into the furnacecontroller each consisting of heating to >1014° C. at a rate of 0.85°C./min and cooling back to growth temperatures below 995° C. at aninitial rate of 0.45° C./min with the cooling rate slowing as growthtemperatures were approached. After 12 programmed temperature spikeswere run, an additional slow cooling period brought the furnacetemperature to 974° C. to ensure complete alignment of the Y-123material with the seed. The furnace was subsequently rapidly cooled toroom temperature at a rate of 2.1° C./min.

While the specific embodiments have been illustrated and described,numerous modifications come to mind without significantly departing fromthe spirit of the invention, and the scope of protection is only limitedby the scope of the accompanying claims.

1. A method for enhancing melt-textured growth of a superconducting crystalline structure comprising: providing a sample in the form of a pellet having a first material capable of exhibiting superconducting properties when subjected to melt-textured growth, providing said pellet with a seed material prior to heating said sample above a peritectic temperature of said first material, cooling said sample below the peritectic temperature of said first material, subjecting said sample to at least one temperature spike wherein said sample is reheated above the peritectic temperature of said first material to a reheating temperature and subsequently recooled below the peritectic temperature of said first material to a recooling temperature to produce a superconducting crystalline structure substantially free of secondary nucleations.
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 3. The method according to claim 2, wherein said first material is a metal oxide.
 4. The method according to claim 3, wherein said metal oxide has a chemical formulation of REBa₂Cu₃O_(7-x), wherein RE is at least one rare earth metal selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb, and wherein x is a number sufficient to provide oxide superconductivity.
 5. The method according to claim 4, wherein RE is Y.
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 7. The method according to claim 6, wherein said seed material has a chemical formula of REBa₂Cu₃O_(7-x), wherein RE is at least one rare earth metal selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb, wherein x is a number sufficient to provide oxide superconductivity, and wherein said rare earth metal is selected to provide said seed material with a higher seed peritectic temperature than the peritectic temperature of said first material.
 8. The method according to claim 7, wherein RE is Sm.
 9. The method according to claim 4, wherein said first material decomposes into a peritectic mixture of the first material comprising a second metal oxide material having a chemical formula RE₂BaCuO₅, wherein RE is at least one rare earth metal selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Nd, La, Pr, and Yb, and a liquid containing the RE, a Ba oxide, and a Cu oxide.
 10. The method according to claim 9, wherein RB is Y.
 11. The method according to claim 2, wherein the step of subjecting said sample to at least one temperature spike comprises subjecting said pellet to a plurality of said temperature spikes, wherein each of said spikes beginning with a second one of said spikes comprises a lower recooling temperature than the recooling temperature of the immediately preceding spike.
 12. The method according to claim 11, wherein a last spike of said plurality of temperature spikes comprises a recooling temperature of at least 35° C. below the peritectic temperature of said first material.
 13. The method according to claim 2, further comprising sintering said pellet at a temperature below the peritectic temperature of the first material prior to the step of heating said sample above a peritectic temperature of said first material.
 14. The method according to claim 13, wherein the step of heating said sample above a peritectic temperature of said first material comprises heating said pellet to a temperature approximately 10° C. above the peritectic temperature of the first material to bring the first material to a semi-molten state.
 15. The method according to claim 14, wherein the step of cooling said pellet below the peritectic temperature of said first material comprises cooling said pellet to a temperature approximately 10° C. below the peritectic temperature of the first material to promote crystalline structure growth.
 16. The method according to claim 15, wherein said reheating temperature is at least approximately 10° C. above the peritectic temperature of the first material and said recooling temperature is at least approximately 10° C. below the peritectic temperature of the first material.
 17. The method according to claim 16, wherein the step of subjecting said sample to at least one temperature spike comprises subjecting said sample to a plurality of said temperature spikes wherein each of said spikes comprises a lower recooling temperature than the recooling temperature of the immediately preceding spike.
 18. The method according to claim 2, wherein said pellet further comprises a modifier to enhance the melting of said first material and the destruction of secondary nucleations within said pellet.
 19. The method according to claim 18, wherein said modifier is selected from the group consisting of cerium, gold, platinum, palladium, ruthenium and silver.
 20. The method according to claim 2, wherein said pellet further comprises a dopant to enhance the destruction of secondary nucleations within said pellet.
 21. The method according to claim 20, wherein said dopant is selected from the group consisting of calcium, lithium, strontium, uranium, zinc, and zirconium.
 22. The method according to claim 2, wherein said pellet is formed via compressing said first material into a die.
 23. The method according to claim 22, wherein said die is substantially hexagonal in cross-sectional shape.
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